The present invention relates generally to image sensors, more particularly to techniques for transferring image signals to readout electronics from image sensors, and even more particularly to techniques for transferring image signals to readout electronics from area image sensors, such as charge coupled devices (CCD""s), for measuring image focus and/or image exposure level.
An image detection system, as for example a digital camera, comprises an image sensor to detect the image and readout electronics for collecting the charge from that image sensor. Among other techniques, modern image sensors can be fabricated using solid state technologies such as those of Charge Coupled Devices (CCD""s). Some image sensors integrate the light incident upon each of its individual elements. For such image sensors, the longer the photosensitive elements are exposed to the image, the stronger the signal obtained. Also for a fixed exposure time in such image sensors, the lower the ambient light, the lower the signal level obtained with an associated lower signal to noise level. Exposure time on the upper end is limited by the fact that the resultant image will be blurred if the source of the image moves appreciably during the time of the exposure.
Typically, an area image sensor comprises a number of adjacent horizontal rows or lines which are comprised of individual photosensitive detection areas referred to as pixels. If the image sensor has the ability to detect various color components of the image, each pixel is further divided into appropriate detection elements which typically detect additive primary colors. The pixels thus arranged detect a two dimensional representation or image of the source of that image.
Image sensors using CCD technology are now used in single frame cameras, referred to as digital still cameras (DSC). Such cameras often include capabilities for automatic focus and exposure level adjustment. There are three image sensor readout architectures currently in common use in commercial DSC""s: frame transfer, interline progressive-scan, and interline interlace-scan. Readout of the image from the image sensor depends upon which architecture is used. In a frame transfer image sensor, horizontal lines of pixels are shifted downward into a horizontal readout shift register one line at a time. While in the horizontal readout shift register, the pixels of each line are read out horizontally one pixel at a time into a detection circuit. If the frame transfer image sensor includes additional storage lines, the time required to transfer a frame is increased accordingly.
For an interline progressive-scan image sensor, the readout is performed by first shifting the image charge from all photosensitive element sites into light protected, vertical interline image sensor registers, shifting this charge down one line at a time into a horizontal readout register, and finally shifting each line out of the horizontal readout register one element at a time into a detection circuit.
The process is similar for an interline interlace-scan image sensor, except that only odd lines or even lines are selected at one time to be shifted out of the photosensitive sites into the vertical interline image sensor registers. Two passes in an interline interlace-scan image sensor are required to read all the image charge out of the image sensor. Other image sensors, in particular those not capable of directly addressing specific, individual pixels may have similar architectures.
Signal to noise ratios available with current image capture devices describe the result of the physical phenomena which limits their ability to obtain proper focus and appropriate exposure settings under low light level conditions. There is an ongoing need for the ability to obtain exposures using digital cameras at lower and lower light levels. Thus, there is a need for an improved image readout technique to increase the signal to noise ratio for image detectors under such conditions.
In addition, automatic focus and the automatic adjustment of exposure level may require the readout of several images prior to detection of the final, focused image. These added image acquisitions increase the total time the image source needs to remain still in order to obtain a clear image. Together, these requirements add to the total time required to obtain a single final image. Present methods have the disadvantage of a lengthy time between shutter release and final image capture. Thus, there is an added need for an improved image readout technique to shorten the time required to perform auto-focus and the determination of exposure level in a digital camera.
In representative embodiments, the present patent document discloses methods and apparatus for measuring parameters such as, but not limited to, focus and exposure level for recording images with an image recording system such as, for example, an electronic single frame digital camera having a frame transfer, interline progressive-scan, or interline interlace-scan charge coupled device (CCD). Techniques are described for combining or binning charge from odd and even rows even when the associated colors detected are different. By such binning, the signal to noise ratio is increased when making such measurements, thereby permitting use under lower light level conditions.
In addition, under certain conditions readout time for focus and exposure adjustment can be reduced which improves the overall shutter button to image capture time. Reducing readout times is especially desirable because multiple focus and exposure readouts may be required to acquire one image. Obtaining a focus or exposure value in fewer scans will result in reduced time to image capture. In particular, in previous interline interlace-scan systems, focusing, setting exposure level, and the exposure of the final image have each included reading the charge from full frames two times eachxe2x80x94once for the odd rows and once for the even rows. In addition, it is often necessary to read out multiple frames in order to set the focus and/or exposure. Using the techniques of charge binning disclosed herein odd and even rows can be readout together.
While the term pixel may have various meanings, it is used herein to mean the smallest, repeated block of photosensitive elements used for image detection in an image sensor. If the image sensor can resolve colors, the pixel includes the smallest, repeated block of color sensitive photosensitive elements used. The pixels are typically located in a two dimensional array comprising a number of adjacent horizontal rows or lines with each line comprised of individual photosensitive detection elements. The pixels thus arranged detect a two dimensional representation or image of the source of that image.
In representative embodiments, the invention is applicable to various types of image sensors including, but not limited to, frame transfer image sensor""s, interline progressive-scan image sensor""s, interline interlace-scan image sensors. It is applicable to image sensors fabricated using CCD technology, as well as other image sensors which generally read approximately a full frame instead of reading only signals from specific pixels.
An interline interlaced-scan image sensor is used as an example in a representative embodiment. Signal to noise level is increased and readout time is reduced for an exposure of the image sensor by combining charge from adjacent photosensitive elements in this embodiment. While the representative embodiment utilizes an image sensor in interline interlace-scan transfer mode as the image sensor, other types of image sensors and other modes of transfer may be used in other embodiments to gain enhanced performance.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.